In heating the mold within a furnace or heating chamber in readiness for pouring, the temperature of the interior of the mold lags behind the outer surface of the mold by reason of the thermal characteristics of the ceramic of the mold. If the furnace is controlled to continue heating at a maximum rate until the interior of the mold reached pouring temperature, the furnace may become so hot as to cause damage to the exterior of the mold by the high temperature developed. Measurement of the interior of the mold has generally been difficult since positioning of a thermocouple at the inner surface of the mold without interfering with the casting has been difficult or impossible.
It is desirable both economically and for mold quality to use the shortest possible heating time. Degradation of the mold may occur during high temperature preheating of the mold in a vacuum especially if such preheating takes an excessive time. Prior to the present invention very long preheat times were required to be sure the mold was completely heated to the desired temperature, particularly the alloy contacting surfaces. This long term heating frequently damaged the mold resulting in inclusions in the castings which are a cause for rejection of the cast part.
In the withdrawal technique, the mold, when poured, is withdrawn from within the furnace past a heat control baffle at such a rate as to cause the solidification front to move upwardly within the alloy at the desired rate for making columnar grained articles at the same time maintaining a steep thermal gradient in the alloy at and directly above the solidification front. In the past, there has been no additional heating of the furnace to maintain this high thermal gradient during the mold withdrawal. There has also been no adequate measure of the rate of cooling of the alloy so that the rate of withdrawal during and after solidification of the alloy has been generally established by trial and error.